1. Field of the Invention
The present invention relates to a video camera apparatus and, more particularly, to an automatic focus adjusting apparatus which is suitable for use in a video camera apparatus.
2. Related Background Art
In a camera such as a video camera or the like, there has conventionally been known an automatic focus adjusting method called a mountain-climbing method whereby a high frequency component in an image pickup signal which is obtained from an image pickup element such as a CCD or the like is extracted and a focus adjusting lens is moved so that the high frequency component becomes maximum, thereby adjusting a focal point. Such an automatic focus adjusting method has advantages such that a special optical member for focus adjustment is unnecessary and a focal point can be accurately adjusted to an in-focus state irrespective of a near or far distance and the like. The automatic focus adjusting method of such a kind of conventional video camera will now be described with reference to FIG. 1.
FIG. 1 is a block constructional diagram of a conventional camera. In the diagram, reference numeral 1 denotes a focusing lens to adjust a focal point. The focusing lens 1 is reciprocated in the direction of an optical axis L by a lens driving motor 2, thereby adjusting a focal point. The light which has transmitted through the focusing lens 1 reaches an image pickup element (CCD) 3 and an image is formed on the image pickup surface of the CCD 3, by which the light image is photoelectrically converted into the electric signal. The electric signal is sampled and held by a sample-hold circuit (CDS) 4. After that, an output signal of the CDS 4 is amplified by an amplifying circuit (AGC) 5 to a proper level. The amplified signal is subsequently supplied to a processing circuit (not shown) and to a band-pass filter (BPF) 7.
The BPF 7 extracts the high frequency component in the video signal. Only the signal corresponding to the portion which has been set into a focus detection area in a picture plane is extracted by a gate circuit 8. The peak value of the extracted signal from the gate circuit 8 is held by a peak-hold circuit 9 at an interval synchronized with a vertical sync signal. After that, peak holding signal is supplied as a focus estimation value to a microcomputer 10. A command signal is supplied to a motor driver 11 and the motor 2 is driven on the basis of the focus estimation value in a direction such that when the focus estimation value is small, namely, in case of a large out-of focusing state, the automatic focus adjusting operation is rapidly executed, and in case of a small out-of focusing state, the automatic focus adjusting operation is slowly performed, and that the focus estimation value is always set to a large value. That is, what is called a mountain-climbing control which has conventionally been well known is executed.
In the above conventional video camera, however, since the automatic focus adjusting operation is performed on the basis of an output of the band pass filter 7, in the case where a contrast, a brightness, or the like of an object extremely differs, it is difficult to similarly execute the operations under all of the conditions.
The above point will now be described with reference to FIG. 3. FIG. 3 is a diagram showing a change in output of the peak-hold circuit 9 when the focusing lens 1 has been searched and moved from the shortest position to the infinite (.infin.) position. In the diagram, a curve A shows output characteristics when an object of an extremely high contrast has been photographed. An extremely high output is generated at the in-focus point. On the other hand, a curve B shows output characteristics when an object of a low contrast has been photographed. A large output is not generated even at the in-focus point.
When the focusing lens 1 is moved so as to be optimally adjusted to the mountain of the curve A, an inconvenience such as hunting or the like occurs in the automatic focus adjusting operation in case of the mountain of the other curve B. On the contrary, when the focusing lens 1 is moved so as to be optimally focused to the mountain of-the other curve B, an inconvenience such that the automatic focusing operating speed is slow occurs in case of the mountain of the curve A.